url size fixes

addurl: Improve message when adding url with wrong size to existing file.
Before the message suggested the url didn't exist.

Fixed handling of URL keys that have no recorded size. Before, if the key
has no size, the url also had to not declare any size, which was unlikely
and wrong, or it was taken to not exist. This probably would mostly affect
keys that were added to the annex with addurl --relaxed.

Command/AddUrl.hs

@@ -124,14 +124,16 @@ perform relaxed url file = ifAnnexed file addurl geturl
 			next $ return True
 		| otherwise = do
 			headers <- getHttpHeaders
-			ifM (Url.withUserAgent $ Url.check url headers $ keySize key)
-				( do
+			(exists, samesize) <- Url.withUserAgent $ Url.check url headers $ keySize key
+			if exists && samesize
+				then do
 					setUrlPresent key url
 					next $ return True
-				, do
-					warning $ "failed to verify url exists: " ++ url
+				else do
+					warning $ if exists
+						then "url does not have expected file size (use --relaxed to bypass this check) " ++ url
+						else "failed to verify url exists: " ++ url
 					stop
-				)
 
 addUrlFile :: Bool -> URLString -> FilePath -> Annex Bool
 addUrlFile relaxed url file = do